Subject matter of the invention is a reagent preparation for binding components of a sample in the form of a tablet, the use thereof for binding or purifying nucleic acids and a method of preparing a suspension of magnetic particles in a sample, and a method of incorporating magnetic particles in a sample.
A problem which frequently arises in the analysis of liquid samples is that the components to be analyzed are present only in very minute amounts. Moreover, the sample also contains numerous particles which are not to be determined but render the determination less accurate. It is therefore expedient to bind the analytes to a solid phase and remove the particles which are not to be determined together with the liquid. The isolated analytes can then be detected at the solid phase. Recently, especially the inner walls of reaction vessels such as tubes have been used as solid phases. Another option is to add a bead to the reaction vessel which is capable of binding the analyte. The bead size is such that the separation of liquid and beads can be accomplished by simple pipetting. Recently, however, continuously operating instruments have been designed where the analyte is bound to magnetic particles, and the bound analyte together with the magnetic particle are separated from the surrounding liquid with the aid of a magnetic field. The magnetic particles are provided with a surface capable of binding an analyte.
These magnetic particle containing reagent preparations are offered in the form of suspensions to which the analyte-containing liquid to be assayed is added by pipetting. These pipetting steps are subject to deviations commonly found in connection with pipetting procedures. Further, pipetting errors are also difficult to trace back.
It was hence an object of the present invention to eliminate the disadvantages found in the prior art and providing magnetic particles which allow easy dosing.
Subject matter of the invention is hence a reagent preparation for binding components in a sample in the form of a tablet comprising a multitude of particles having a surface to which the components can essentially completely bind and excipients. Another subject matter of the invention is the use of these reagent preparations and a method of preparing magnetic suspensions.
Components are understood to be particulate or molecular material. This includes especially cells, e.g. viruses or bacteria, but also isolated human or animal cells such as leukocytes, then also immunologically active low and high molecular chemical compounds such as haptens, antigens, antibodies, and nucleic acids. Particularly preferred are nucleic acids such as DNA or RNA.
Samples as understood in the invention are for example clinical specimen such as blood, serum, mouth wash liquid, urine, cerebrospinal fluid, sputum, stool, punctate, and bone marrow samples. The sample can also stem from areas such as environmental analysis, food analysis or molecular-biological research, e.g. bacterial cultures, phage lysates, and products of amplification processes such as PCR.
A tablet as understood in the invention is a solid, formed body, preferably in the form of a disk or a more or less perfectly shaped sphere. Other similar embodiments are also conceivable. Tablets of this kind are commonly known from drugs. A tablet preferably has a defined weight which exceeds 5 mg.
A magnetic particle is a particle made of a material which can be attracted by a magnet, i.e. ferromagnetic or superparamagnetic materials. The invention prefers in particular superparamagnetic particles, especially those that are not premagnetized. Premagnetization as understood here is a process of bringing a material into contact with a magnet to increase resonance. Magnetide (Fe3O4) or Fe2O3 are particularly preferred. A magnetic particle is, however, also understood to include materials which contain (smaller) magnetic particles. This includes in particular Iriodin 600 a pigment which is commercially available from Merck (Darmstadt, Germany). The invention prefers in particular particles with an average grain size of less than 100 xcexcm. A particularly preferred grain size ranges between 10 and 60 xcexcm. The preferred grain distribution is relatively homogeneous; in particular, there are almost no particles smaller than 10 xcexcm or larger than 60 xcexcm. Particles which satisfy this requirement are described for example in WO 90/06045.
An essential element of the invention is the fact that magnetic particles have a surface to which components can bind. This binding can either be specific or relatively non-specific. Specific binding can be achieved by making use of a binding-specific interactions, e.g. antibodies and antigens, antibodies and haptens or complementary nucleic acids. A combination of these interactions is also possible.
A known method of modifying a surface is, for example, the coating of particles with a streptavidin layer. It is thus possible to generate a universal matrix to which specific components can be bound from the sample via conjugates of biotin and a certain antibody, hapten or nucleic acid. The expert, especially one from the field of immunoassays, is familiar with corresponding embodiments.
A relatively unspecific binding is the interaction between a glass-like surface and nucleic acids. The binding of nucleic acids from agarose gel in the presence of sodium iodide in ground flint glass is known from Proc Natl Acad USA 76, 615-619 (1979). U.S. Pat. No. 2,233,169 describes magnetic particles whose glass portion can bind nucleic acids.
The invention proposes that the component to be determined bind essentially completely to the magnetic particles. The expert can easily determine the necessary amount of particles by varying the amount of magnetic particles to be added. As understood in the invention, an essentially complete binding means binding of more than 60%, particularly preferred more than 90% of the component to be bound found in the sample.
Excipients essentially serve to maintain the shape of the tablet, i.e. to link the magnetic particles to form a tablet. Preferred excipients of the invention are those which dissolve rapidly in the sample where the reaction is to take place. As preferred liquid samples are aqueous solutions, it is possible to use those excipients that are usually employed in the manufacture of drugs. Polyethyleneglycol (PEG) and polyvinylpyrrolidon (PVP) are particularly preferred.
DE-A-4406139 describes a magnetic depot drug with improved absorbance of the active components. The tablet contains a disk-like magnet and the active component is released over period of several hours.
The International Journal of Pharmaceutics 119, 47-55 (1995) also describes a tablet with a delayed release of the drug.
STP Pharmasciences Vol 4, 425-430 (1994) describes the manufacture of ferrrite-containing magnetic tablets and their administration to dogs.
Moreover, the tablet of the invention can also contain stabilizing reagents. In a preferred manner, sugars such as D-mannite, trehalose, and sorbite are added.
Surprisingly, magnetic particles, especially those with a glass surface, can be stored in the form of a tablet without visible hydrolysis of the glass and hence without visible elution of the iron from the magnetic portion.
The magnetic particles are preferably glass magnet pigments or polymer magnetic beads or other magnetic particles with a size ranging between 0.1 xcexcm and 100 xcexcm; e.g. those described in DE 19520390.
The preparation can also contain additives to facilitate the binding process of the components. This includes specificity enhancing substances like the above mentioned conjugates; but also substances which modify the sample properties such that the binding of the components to the surface is facilitated. When nucleic acids are used these are chaotropic salts such as guanidinium hydrochloride, sodium iodide, sodium perchlorate or the like. Chaotropic salts of this kind are known from Anal. Biochem. 121, 382-387 (1982) and DE-A 3734442.
The reagent preparation can also contain reagents which convert the components into a form which basically enables a binding process. This includes reagents to lyse compartments, e.g. cells, which contain nucleic acids. Such a reagent is, for example, proteinase K or the above chaotropic salts.
The reagent preparation can also contain pH buffer substances and reagents for dissolving links such as hydrogen bridges, hydrophobic and ion links as well as reagents for the specific detection of substances or indicators as they are known with components of immunoassays.
The following composition has proven to be feasible for a preferred tablet:
The tablet of the invention can of course also contain other components, e.g. inert filling agents; the total amount acids up to 100%. The percentages given are weight percentages.
The reagent preparation of the invention in the form of a tablet can be manufactured corresponding to other drugs in tablet form. To accomplish this, all necessary components are thoroughly mixed and aliquots are tabletted in a tablet press. This is accomplished in particular by applying pressure. Tablets of the invention can, however, also be obtained by granulating the mixture of components. For this purpose, a certain amount of the dry mixture is granulated with a solubilizing liquid. Then liquid is again withdrawn from the so obtained granulate. Uniform grain size can be obtained by sieving the granulate.
These manufacturing processes entail a very low coefficient of variation of the tablet weight and hence a high reproducibility when dosing the reagent in the practice. Erroneous dosing is then reduced and easier to trace back. The tablets of the invention can be rapidly dissolved, preferably in less than 30 sec., particularly preferred in less than 1 to 10 sec. while the magnetic particles can be easily and readily dispersed. Tablet form is also expedient with respect to storage. Dosing can even be accomplished manually with the aid of a tablet dispenser. Adulterations which occur in suspensions and are caused by sedimentation of particles have not been observed.
Another subject matter of the invention is the use of the reagent preparation for binding nucleic acids. To accomplish this, the reagent preparation is added to the sample and incubate until (1.) the tablet has dissolved and (2.) the nucleic acids are essentially completely bound to the surface. The tablet can be mechanically moved, if necessary. This increases both the dissolving rate of the tablet and the binding rate of the components.
Another subject matter of the invention is the use of the reagent preparation for purifying nucleic acids. To achieve this, the magnetic particles and the nucleic acids bound thereto are separated from the surrounding sample liquid. This is advantageously accomplished in that a magnetic field is applied to retain the magnetic particles in a vessel or at a defined site of the apparatus; then the sample liquid is removed (by e.g. pipetting or displacement) and, if desired, one or several washing steps with other liquids are performed. If desired, the bound nucleic acids can be separated again from the magnetic particles when suitable conditions are applied. In the case of a glass-like surface, these are low-salt conditions, i.e. the salt contents of the elution solution is less than 100 mmol/l.
Another subject matter of the invention is a method of preparing a suspension magnetic particles in a sample comprising the steps of adding to the sample a tablet containing magnetic particles and soluble excipients and moving the tablet in sample, preferably with the aid of a movable magnetic field. The magnetic field can be moved in that a magnet in the vicinity of the sample is moved back and forth such that the magnetic particles are subject to continuous movement. It is, however, also possible that the vessel containing the sample with the tablet and the magnetic particles is moved with respect to the magnet.
Yet another subject of the invention is a method of incorporating magnetic particles in a sample comprising the steps of providing a dispenser which contains a multitude of magnetic particle-containing tablets and activating the dispenser to release a tablet. Dispensers for providing tablets are commonly used when administering drugs in the form of tablets. They can be used manually for dosing procedures in the method of the invention. It is not absolutely necessary to release only one tablet per sample. It is also possible to release a defined number of tablets, e.g. between 2 and 10, depending on the intended use in the sample.
The following example explain the invention greater detail: